Pyrolysis of solid carbonaceous materials such as for example biomass or carbonaceous waste materials is carried out by heating the solid material under non-oxidising conditions. The solid carbonaceous materials is first heated and then decomposed into gaseous compounds. Condensable gases formed during the pyrolysis are cooled to obtain a liquid phase called pyrolysis oil. Conventionally, pyrolysis has been carried out with relatively long residence time of the solid feedstock particles in the pyrolysis reactor. This leads, however, to undesired secondary cracking reactions and a relatively low yield of pyrolysis oil. A higher yield of pyrolysis oil, higher efficiency and less secondary reactions are achieved in so-called flash pyrolysis. In a flash pyrolysis process, relatively small particles of the feedstock, typically with a diameter in the order of a few millimeters, are fed to a reactor and heated for a relatively short time under continuous movement of the particles. The gaseous phase formed is cooled and condensed before extensive secondary reactions occur. Known suitable reactors for flash pyrolysis, also referred to as fast pyrolysis, include cyclone and swirl reactors.
In WO 01/34725 for example is disclosed a cyclone reactor for flash pyrolysis. A feed stream comprising feed particles and a carrier gas is introduced into the cyclone reactor near the top of the reactor. A product stream comprising solids is discharged from the reactor at the bottom and a gaseous stream steam is discharged at the top of the reactor.
In cyclone-type reactors, the solid material to be pyrolysed has a certain, finite residence time since the particles are forced to the bottom outlet of the reactor due to gravity forces. In case of a feedstock comprising particles above a certain critical size, such larger particles will be discharged from the reactor before they are sufficiently converted. In the art, swirl or cyclone pyrolysis reactors or operating modes for such reactors have been proposed to increase the residence time of larger particles. In WO 01/34725 for example is mentioned that the cyclone reactor may be operated ‘bottom-up”. A feed stream comprising feed particles and a carrier gas is then introduced into the cyclone reactor near the bottom of the reactor. Solid particles move upwards with a speed depending on the force balance of gravity and drag. The solid particles are discharged from the cyclone reactor via an inner cyclone placed in the outer cyclone and thus leave the reactor at the bottom end (see FIG. 3a of WO 01/34725). In WO 01/34725 is further mentioned a swirl-type reactor (see FIG. 3b of WO 01/34725) wherein a feed stream comprising feedstock particles and a carrier gas is introduced into a swirl tube near the bottom of the tube and solid particles and a gaseous stream are exiting the swirl tube as separate streams at the top of the tube.
In DE 3814723 is disclosed a swirl-type pyrolysis reactor wherein large particles have a longer residence time than small particles. In the reactor of DE 3814723, a feed stream comprising solid feedstock and a carrier gas is tangentially introduced at the bottom of an annular swirling reaction vessel. At the top of the annular swirling reactor, the swirling reaction mixture comprising solid and gaseous material is forced to flow down into an inner annular channel acting as a cyclone. In the cyclone, solid or fluid particles are separated from the gaseous stream. At the bottom of the inner annual channel, solid particles are recycled into the (outer) annular reaction space to undergo another reaction cycle. Gaseous product is discharged from the reactor via an inner exit tube.
Although reactors for pyrolysis processes wherein the residence time of large particles are increased are known, there is still a need for improvement for such processes, in particular for improved control of residence time of feedstock particles as a function of the particle size and of improved control of residence time of gaseous products formed, in order to avoid over-reaction and secondary cracking reactions.